The present invention relates to inkjet printers and, in particular, it concerns print head configurations for such printers.
Impulse inkjet systems are well known in the art. They generally fall into two categories: continuous systems and drop on demand systems. Continuous inkjet systems operate by continuously ejecting ink droplets at high frequency, some of which are deflected by suitable means prior to reaching the substrate being imprinted, allowing the undeflected drops to form the desired imprinting pattern. Drop on demand systems eject drops selectively as required.
Drop on demand inkjet systems may, in turn, be divided into two general categories on basis of the principle of ejecting the droplets. Most systems in use today are the thermal bubble jet type wherein the ejection of ink droplets is effected by boiling of the ink.
Thermal bubble system, like the one disclosed in Japanese patent application No. 61-59913, includes thermoelectric heating elements. Actuation of a specific element causes the ink in that cavity to boil which causes a sudden rise in pressure, thus ejecting an ink drop through the nozzle. Bubble jet printing systems are advantageous in the ease of their miniaturization. On the other hand, they suffer some disadvantages relative to piezoelectric systems. One such disadvantage is the short useful life of the heating elements due to the high stresses imposed on the resistor protecting layer. In addition, it is relatively difficult to control precisely the volume of the drop and its directionality.
Still another drawback is the low frequency of printing signals which may be applied consistently to the printing head. Still another drawback of the thermal bubble system is that it is limited to special ink formulations which can withstand boiling temperature without mechanical or chemical degradation.
Other drop-on-demand inkjet systems use piezoelectric crystals which deform when a voltage is applied to them, thereby causing the ejection of a drop of ink from an adjoining ink cavity, as will be shown below. Ink is fed to the cavity through a restricted inlet opening, and leaves the cavity through a nozzle. The relative fluid impedance of the restricted inlet opening and the nozzle is such that a suitable amount of ink exits the outlet nozzle during the bending of the diaphragm. Replenishment of the cavity with ink is a result of the capillary action of the ink meniscus in the nozzle and the return motion of the diaphragm. The time taken to replenish depends on the fluid impedance.
In contrast to thermal bubble systems, piezoelectric drivers are not required to operate at elevated temperatures, allowing them to accommodate a much wider selection of inks. Furthermore, the shape, timing and duration of the driving pulses are more easily controlled. Finally, the operational life of the piezoelectric crystal and hence the piezoelectric head is much longer.
Piezoelectric crystal drop-on-demand print heads are well known in the art. Some illustrative examples of such developments include U.S. Pat. Nos. 4,730,197 and 5,087,930. These patents disclose a construction having a series of stainless steel layers. The layers are of various thickness and include various openings and channels. The various layers are stacked and bonded together to form a suitable fluid inlet channel, pressure cavity, fluid outlet channel and orifice plate.
The systems disclosed in the above referenced patents illustrate the use of a fluid inlet channel having a very small aperture, typically 100 microns or less. The use of a very small aperture is dictated by the need to limit the back flow from the ink cavity during ejection of a drop. On the other hand, it is problematic in that the small aperture is susceptible to clogging during the bonding of the layers as well as during normal operation of the print head. Additionally, the techniques used for forming the openings in the orifice plate, which typically include punching, chemical etching or laser drilling, require that the thickness of the orifice plate be equal to, or less than, the orifice diameter, which is itself limited by resolution considerations to about 50 microns. Finally, any air bubbles or other gaseous substance trapped in the flow channels cannot easily be purged, and because bubbles are compressible, their presence in the system can have detrimental effects on the system performance.
Piezoelectric elements are used in inkjet heads in various configurations, each having its implications for the cavity construction. Some examples are: a layered type, as shown schematically in FIG. 53 of U.S. Pat. 5,666,141, in which a rod shaped layered element extends longitudinally as a result of voltage applied to the electrodes, causing a pressure surge in the ink cavity. Another conventional configuration, known as the bimorph-cantilever type, is shown schematically in FIG. 54 of U.S. Pat. No. 5,666,141. In this case, two electrodes are cemented to a piezoelectric element forming a thin leaf. A voltage applied to the electrodes causes the leaf to bend, thereby ejecting a single drop. In more recently developments, the piezoelectric element is typically cemented to a thin plate forming a diaphragm located above the ink cavity.
Two approaches are used to achieve full print coverage of the printed substrate: the conventional construction uses a small printing head containing a limited number of cavities and nozzles (sometimes as low as a single nozzle), each nozzle printing a specific row. To achieve full coverage the printing head is being moved to-and-for while ejecting ink droplets. Each movement of the printing head corresponds to a strip of printed lines, typically one for each nozzle in the head. The printed substrate is also moved forward in steps, the width of the step depending on the number of printing nozzles. This mechanism is commonly used in desk printers and the like. Its main disadvantages are the limited printing speed and the high noise level it produces.
The second approach, to which the present invention primarily relates, is the full array approach. According to this approach, each pixel across one dimension of the substrate is covered by a specific nozzle. Although this approach necessitates a large number of nozzles, it can achieve very high printing speed and silent operation.
In order to provide high nozzle densities over a small area, conventional inkjet print heads are typically formed on silicon or ceramic wafers by use of masking or etching techniques. The use of such wafers renders the structures uneconomical for implementing large two-dimensional arrays of cavities.
As an alternative to the use of a constricted fluid inlet channel aperture with its associated problems mentioned above, it has been suggested that suitable ink flow impedance could be combined with advantageous filtering properties by passing the ink into the cavities through a porous layer. The principles of this approach are described in the parent application of this application, now issued as U.S. Pat. No. 5,940,099 to the present applicant. In order to achieve high quality uniform printing, it is important that the ink supply to the porous layer should uniform with respect to the cavities. However, the parent patent does not address details of how to achieve uniformity of ink supply across the porous layer for large two-dimensional nozzle arrays.
A further issue relating to inkjet print head design is the choice of material for the front face of the printing head. For a range of reasons including mechanical and chemical properties and ease of production, polyimide compositions are frequently preferred. However, it has been found that a polyimide front surface has a tendency to collect small splashes of ink and other residues, leading to inferior printing quality and reduced reliability.
There is therefore a need for an inkjet print head which provides an improved ink supply through a porous layer to a plurality of ink cavities. It would also be highly advantageous to provide an inkjet print head with a polyimide-based nozzle plate which would avoid build-up of ink on the front surface.
The present invention is an inkjet print head.
The present invention provides an inkjet printing head capable of high printing speed, high reliability, and having the ability to use many kinds of ink formulations.
In most preferred implementations, these properties are achieved, amongst other features, by using an ink supply layer including porous material. Preferably, the porous material includes sintered material, most preferably sintered stainless steel.
The ejection of an ink drop is accomplished as follows: a pressure pulse is imparted to a volume of ink in an ink cavity through the deflection of a thin deflection plate located on top of the ink cavity. The plate is deflected downwards by the action of a piezoelectric element whenever a voltage is applied across its electrodes, one of which is in electrical contact with the metallic deflection plate. The pressure pulse created by the downwards bending of the deflection plate drives the ink through the nozzle, thus causing the ejection of an ink droplet of specific size.
When the piezoelectric element is de-energized it returns to its equilibrium position, reducing the pressure in the ink cavity and causing the meniscus at the end of the nozzle to retract. The retracted meniscus generates a capillary force in the nozzle which acts to pull ink from the porous material into the cavity. The refilling process ends when the meniscus regains its equilibrium position.
A key element in preferred implementations of a print head according to the present invention is the presence of the porous material which acts as hydraulic linkage between the ink main supply and the individual ink cavities. Proper selection of the porous material, grain size, pore size, type of alloy and the machining processes imparts the plate with proper flow impedance values as well as making it an efficient filter.
It will be readily appreciated that in order to achieve high drop ejection rate, the time required to refill the ink cavity following ejection of a drop must be as short as possible. The refilling time can be reduced by reducing the restriction to flow into the cavity. However, reduction of the restriction to inflow tends to increase the adverse effects of cross talk, i.e., the undesired interaction between separate ink cavities.
The optimization of the system in terms of conflicting requirements of low cross talk and high refill rate can be effected through the judicious selection of a porous material having optimal characteristics for the intended application, taking into account, in addition, the viscosity of the ink and the nozzle geometry.
As stated above, the optimal balance between the in-flow of ink into the ink cavity and out-flow to neighboring cavities depends also upon the ink viscosity and nozzle dimension. The lower the viscosity of the ink, the faster the refilling rate of the ink cavity will be, but the more pronounced will be the cross talk between separate cavities. Also, the smaller the outlet nozzle diameter, the more pronounced will be the capillary action of the nozzle and hence, the higher the refilling rate.
Inkjet print heads are generally designed so that the dimensions of the ink channels have acoustic impedance which is optimal for a specific ink of a given viscosity and for a specific nozzle diameter. If it is desired to use a print head with a different nozzle diameter and/or with ink of a different viscosity, conventional print head channels must be redesigned to accommodate the new nozzle diameter and/or different viscosity. By contrast, use of a porous material according to the present invention, makes it possible to preserve the same print head geometry and structure even when ink of a different viscosity and/or when a different nozzle geometry are to be used.
The optimization of the acoustic impedance of the channels can be effected merely through the proper selection of a suitable porous material having suitable characteristics, such as a suitable micron grade.
Apart from the ability to optimize the print head without the need to redesign the flow channels, use of a porous material according to the present invention eliminates the small, and easily clogged, ink inlet apertures used to supply ink to the cavities of conventional inkjet print heads.
Still another advantage offered by the use of the porous material according to the present invention is the material ability to act as filter, thereby reducing, or even completely obviating, the need for special filtration of the in-flow ink.
Finally, the fabrication of print heads including porous material according to the present invention can be effected using simple production techniques without the need for complex and expensive micro-machining.
According to the teachings of the present invention there is provided, an inkjet printing head comprising: (a) a nozzle layer defining a plurality of ejection nozzles; (b) a cavity layer having a plurality of apertures, each aperture being positioned to correspond to one of the ejection nozzles so as to at least partially define a corresponding ink cavity; and (c) an ink supply layer having a front surface associated with the nozzle layer and a rear surface associated with the cavity layer, the ink supply layer being formed with a plurality of connecting bores from the rear surface to the front surface, each connecting bore being aligned so as to connect between a corresponding one of the ink cavities and a corresponding one of the ejection nozzles, wherein the ink supply layer is formed from a porous material having a multitude of small interconnected pores so as to allow passage of ink therethrough, wherein the ink supply layer additionally features: (i) a pattern of ink distribution channels formed in the front surface, and (ii) at least one ink inlet bore passing from the rear surface to the front surface and configured so as to be in direct fluid communication with at least part of the pattern of ink distribution channels, the pattern of ink distribution channels and the at least one ink inlet bore together defining part of an irk flow path which passes from the rear surface through the at least one ink inlet bore to the pattern of ink distribution channels on the front surface, and through the porous material to the plurality of ink cavities.
According to a further feature of the present invention, the at least one ink inlet bore is implemented as a plurality of ink inlet bores spaced around a peripheral edge of the ink supply layer.
According to a further feature of the present invention, each of the ink inlet bores is angled such that the intersection of the ink inlet bore with the rear surface of the ink supply layer occurs at a position nearer to the peripheral edge than the intersection of the ink inlet bore with the front surface of the ink supply layer.
According to a further feature of the present invention, there is also provided a rigid casing rigidly attached to the nozzle layer, the ink supply layer and the cavity layer, the rigid casing being formed with a plurality of ink conduits, each of the plurality of ink conduits being configured to supply ink to a corresponding one of the plurality of ink inlet bores of the ink supply layer.
According to a further feature of the present invention, the plurality of connecting bores define an array on the front surface having two row directions, and wherein the pattern of ink distribution channels includes a plurality of channels deployed substantially parallel to one of the row directions and interposed between adjacent rows of the connecting bores.